1. Field of the Invention
This invention relates to a fullerene network of carbon and more specifically to fullerene tubes. The fullerene tubes may have a diameter of several nanometers and sufficient length to be utilized as fibers. The invention also relates to methods for making fullerene tubes and fullerene fibers.
2. Description of the Prior Art
Carbon fibers have long been known and many methods for their production have been developed, see, for example, M. S. Dresselhaus, G. Dresselhaus, K. Suglhara; I. L. Spain, and H. A. Goldberg, Graphite Fibers and Filaments, Springer-Verlag, New York (1988). However, while these conventional carbon fibers are easily made very long, the graphite sheets within their structure are either not closed tubes, or do not extend continuously along the length of the fiber, or both. The result is sharply decreased tensile strength, electrical conductivity, and chemical resistance compared with what one expects for a fiber where the carbon is bonded in a perfect fullerene network.
Fullerenes have recently been identified as the third form of pure carbon, and the only molecular form of pure carbon yet discovered, see "Fullerenes," Curl, R. F. and Smalley, R. E., Scientific American, October, 1991, pp. 54-63, incorporated herein by reference, and references cited therein.
A fullerene network can be visualized as a single sheet of graphite curled around on itself by the inclusion of 12 pentagonal ring defects in the otherwise perfect hexagonal lattice of a graphite sheet so that the edges connect to form a hollow spheroid. A fullerene network constitutes an atomic-thickness carbon membrane which is impermeable to atoms and molecules under ordinary conditions. Atoms trapped inside a fullerene network are therefore largely immune to chemical attack from the outside of the closed spheroid. The most symmetrical of these structures, (@C.sub.60), "buckminsterfullerene" has the perfectly icosahedral structure of a soccerball, but many other forms are possible as well, such as (@C.sub.70), which has a more elongated shape similar to a rugby ball.
Each carbon atom in an all-carbon fullerene network is bonded to three other carbon atoms. The fullerene network forms a molecule with a cage-like structure and aromatic properties. All-carbon fullerene networks contain even numbers of carbon atoms generally ranging from 20 to 500 or more.
Larger fullerenes are known as well, with many hundreds of carbon atoms bonded together in a fullerene network, and hyperfullerenes may be prepared wherein one closed fullerene network is contained within a second larger closed fullerene network, contained in turn in yet a larger closed fullerene network resulting in an onion-like structure. While these giant, hyperfullerene spheroidal carbon molecules are currently thought to be the most stable forms of fullerenes in terms of cohesive energy per carbon atom, other shapes are possible. In particular, the shape of a fullerene network can be tubular, comprising six pentagons arranged with hexagons on one end of the tube to form a hemispherical end cap connected to a long hollow tube of hexagons, and a final set of six pentagons and more hexagons connecting a second hemispherical end cap to seal the opposite end of the tube. Tubular fullerene networks within larger fullerene networks are also possible, but the tubular fullerene networks known in the prior art generally have lengths of less than 10 microns.
The molecular structure for buckminsterfullerene was first identified in 1985, see NATURE, "C.sub.60 : Buckminsterfullerene", Kroto, H. W., Heath, J. R., O'Brien, S. C., Curl, R. F. and Smalley, R. E., Vol. 318, No. 6042, pp. 162-163, Nov. 14, 1985. The process for making fullerenes described therein involves vaporizing the carbon from a rotating solid disk of graphite using a focused pulsed laser. The carbon vapor was then carried away by a high-density helium flow. That process produced generally spherical fullerenes having 60 carbon atoms although clusters of up to 190 atoms are described. Only microscopic quantities of fullerenes were produced.
The fullerene yield utilizing laser vaporization of carbon was improved by providing a temperature controlled space for the carbon atoms in the carbon vapor to combine in a fullerene structure, see, "Fullerenes with Metals Inside," Chai, et al., J. Phys. Chem., Vol. 95, No. 20, pp. 7564-7568 (1991). Chai et al. describe fullerenes having 130 carbon atoms and describe the possible coalescence of buckminsterfullerene molecules into cylindrical "bucky tubes." Chai et al. do not describe fullerene tubes having more than 200 carbon atoms and describe only coalescence triggered by a laser or an electron beam as a possible way to form the tubes.
Another method of making fullerenes was described in J. Phys. Chem. "Characterization of the Soluble All-Carbon Molecules C.sub.60 and C.sub.70," Ajie et al., Vol. 94, No. 24, 1990, pp. 8630-8633. The fullerenes are described as being formed when a carbon rod is evaporated by resistive heating under a partial helium atmosphere. The resistive heating of the carbon rod is said to cause the rod to emit a faint gray-white plume. Soot-like material comprising fullerenes is said to collect on glass shields that surround the carbon rod. The fullerenes described have 84 or fewer carbon atoms.
Another method of forming fullerenes in greater amounts is described in "Efficient Production of C.sub.60 (Buckminsterfullerene), C.sub.60 H.sub.36 And The Solvated Buckide Ion," Haufler, et al., J. Phys. Chem., Vol. 94, No. 24, pp. 8634-8636 (1990). The fullerenes described have 70 or fewer carbon atoms and are produced when carbon is vaporized in an electrical arc and the carbon vapor condenses into fullerenes.
Short (micron) lengths of imperfect forms of such fullerene fibers have recently been found on the end of graphite electrodes used to form a carbon arc, see T. W. Ebbesen and P. M. Ajayan, "Large Scale Synthesis of Carbon Nanotubes," Nature Vol. 358, pp. 220-222 (1992), and M. S. Dresselhaus, "Down the Straight and Narrow," Nature, Vol. 358, pp. 195-196, (16 Jul. 1992), and references therein. A similar technique was discussed by Roger Bacon, "Growth, Structure, and Properties of Graphite Whiskers," Journal of Applied Physics, vol. 31, no. 2, pp. 283-290 (1960), although the early experiments were operated at high inert gas pressures (95 atm) where thicker carbon "whiskers" are most abundant. With modern high resolution electron microscopes, and the awareness that closed carbon fullerene networks form in abundance in carbon arcs, multiwalled fullerene-like tubes were found to grow readily off the end of such graphite electrodes, and their yield at optimum pressure (near 500 torr Helium) has been found to be quite substantial. See Sumio Iijima, "Helical Microtubules of Graphic Carbon," Nature, Vol. 354, pp. 56-58, (7 Nov. 1991).
High electric fields generated on electrodes with a small radius of curvature can result in the formation of fine carbon whiskers growing out of the electrode. It has long been known that microneedles composed mostly of carbon are formed by the polymerization of hydrocarbons in the high electric field around thin wires of metals such as of tungsten, and it is known that resistively heating these metal wires to temperatures near 1200.degree. C. during growth of the microneedles or whiskers results in a straighter, more graphitic morphology with the graphite planes somewhat aligned along the whisker axis. See B. Ajaalan, H. D. Beckey, A Maas and U. Nitschke, "Electron Microscopical Study of Pyro-Carbon Microneedles Grown by High Field Pyrolysis", Applied Physics, vol. 6, pp. 111-118 (1975). While these carbonaceous whiskers are not fullerene fibers, their production under such circumstances suggests that high electric fields may be useful.
U.S. Pat. No. 4,663,230 describes a carbon fibril having an outer region of multiple essentially continuous layers of ordered carbon atoms and a distinct inner core region, each of the layers and core disposed substantially concentrically about the cylindrical axis of the fibril. The diameter of the fibril is described as 3.5 to 70 nanometers and the length 100 times greater than the diameter.
While the above-described methods of forming fullerenes have, on occasion, formed very short tubular fullerene-like structures, the prior art does not describe any methods known for making continuous fullerene fibers of lengths longer than a few microns.